In a typical telephone network, such as the public switched telephone network (PSTN), signals carrying data travel bi-directionally through the network (i.e., from a transmitting point to a receiving point and from a receiving point to a transmitting point). Echo is one result of this bi-directionality characteristic of the network. Echo is an attenuated and distorted replica of a signal, which can be caused by the transmission and reception of the signal on a common line. For example, in a telephone network that involves both 4-wire and 2-wire links, echoes arise due to impedance mismatches in a hybrid that converts the 4-wire to the 2-wire link. Echo is acceptable if a small delay is present since telephone users are accustomed to hearing an immediate echo. However, echo can become a larger problem as a signal path delay increases, such as within a long distance link with an even longer delay.
One approach to solve this problem is to implement an echo canceller at the network interface. A standard echo canceller performs an adaptive algorithm, which estimates a model (impulse response) of the network echo path. The echo canceller may be positioned to receive return signals from a network and adaptively subtract a replica of an estimated echo signal from the return signal in order to achieve echo cancellation. The echo canceller can continuously adapt to varying echo signals and modify its model of the estimated echo signal path accordingly.
It may be desirable for echo cancellers to have the following fundamental features: rapid convergence, subjective low returned echo levels during single talk, and low divergence during double talk. Real echo paths in real networks make this more than challenging. Sometimes it is impossible to meet sufficient performance criteria in order to completely eliminate echo from a user's listening experience. Most of these “impossible” situations are brought about by certain network conditions, certain mixed network topologies, or even certain network configurations.
The ITU-T has worked to diligently define the standard network echo canceller. (reference is made to ITU-T G.165/G.168). A standard echo canceller designed to meet the fundamental features probably will perform properly for echo loss (e.g., amount of attenuation of echo signals provided by the network) of 6 decibel (dB) or greater. For echo loss less than 6 dB, standard echo cancellers may still work but with degraded performance. (The difference amount of attenuation of the echo signals provided by the network is known as Echo Return Loss (ERL) and is measured in dB.)
The above standards define the terminology to describe a typical echo scenario in the Public Switch Telephone Network (PSTN). Because real applications are moving to hybrid networks (Circuit and Packet switched), there are certain requirements for echo cancellation on local networks that, until now were thought to be unnecessary. This provides an opportunity to extend and leverage existing standard methods to provide a novel solution to current mixed network (converged network) issues. The goal is improving performance in environments that have often been neglected or have had reduced performance expectations associated with them.
However, distinguishing echo signals from desired signals can be difficult due to double-talk situations. Double-talk occurs when two people, one on the receiving side of the network and one on the transmitting side of the network, speak simultaneously. The dual transmission and reception of signals disrupts echo canceller adaptation, and as a result, the echo canceller may perform poorly if a robust double-talk detector is not employed. When no double-talk exists, the echo canceller can properly adapt its model of the echo signal path since the echo canceller only receives a signal that contains the echo. But during double-talk, the receiving side also transmits signals along a return signal path. Therefore, the echo canceller receives both return echo signals and signals transmitted from the receiving end simultaneously, and therefore may adapt improperly. Thus, a problem arises if the echo canceller erroneously adapts its model of the estimated echo signal according to a transmitted signal rather than according to an echo signal. The echo canceller may begin to distort transmitted signals. And for echo cancellers that employ Non-Linear Processors (NLP) to improve echo canceller performance, these challenges become especially important. The ERL used when employing such echo cancellers typically has a minimum value of 6 dB. It should be noted that 6 dB is a typical worst case value encountered for most networks, and most current networks have typical ERL values larger than this.
To properly determine double-talk conditions, to allow decision logic to decide when and when not to allow adaptation, and to correctly engage an NLP algorithm, an ITU-compliant echo canceller typically requires a 6 dB difference in power levels between transmitted signals and associated return echo signals. Therefore, typical echo canceller implementations used in the industry do not operate robustly and well-behaved (e.g., performance equal to or better than the standard and stable echo canceller) if less than 6 dB ERL is present. Because standard network echo cancellers require 6 dB of ERL to function properly, they are typically unsuitable for use within networks that do not provide 6 dB of ERL. But, networks that have less than 6 dB of ERL also have echo path delays that are so small such that the side-tone (i.e., receiver amplification loop from microphone to speaker) of typical analog telephones would cover this echo, and the echo canceller may be unnecessary for these small delay path networks. However, recently it has become important, because of mixed networks (circuit switched and packet switched such as ATM, Frame Relay, IP, Cellular, Satellite, etc.) to provide echo cancellation in areas where ERL is less than 6 dB and the end-to-end delay through this mixed network is large enough to mandate robust echo cancellation in order to meet the performance expectations of a traditional circuit switched network.